In the 1940 to 1942 period the allies had to take into account the possibility of the battleship centric French Fleet being used by the axis.
US Maritime Commission record 5,777 ships built 1939 to 1945, 56,291,678 Deadweight Tons, 39,919,523 Gross Register Tons, 22,218,130 Light Displacement Tons. It should be noted the US maritime commission and USN provided funds for shipyard upgrades, the Maritime commission orders for new slipways stood at 51 as of January 1941, rising to 314 by June 1942, of which 273 were ultimately built, 98 by the end of 1941, 248 by the end of 1942. The first new slipway was in service in April 1941. According to the War Production Board report July 1940 to August 1945, the Maritime Commission program built 50 escort carriers 336,500 tons, the USN program another 34 of 388,120 tons plus 27 conversions. The USN program built 504 Destroyer escorts of 651,725 tons. The Maritime Commission states it built 58 CVE, 4 at Ingalls, 4 at Western Pipe and Steel, 50 Kaiser Vancouver. Western Pipe built HMS Attacker, Fencer, Stalker, Striker while Ingalls built HMS Battler, Hunter, Chaser, Pursuer. End August 1945 the RN/RCN/RAN etc. had 39 CVE (Including British built) after losing 5, 78 US DE after losing 14, the USN had 72 CVE after losing 6, 361 DE after losing 13, plus another 94 DE that became APD.
Since it takes over 2 years between framing requirements, design, construction and training the crew for even a Destroyer Escort it is clear the USN needed the designs pre war and needed them in production, only that which is in production can go into mass production. It should be noted no one was going to build large numbers of small naval vessels until very close to a war, since the money and manpower requirements would heavily inhibit peace time budgets. It is a very difficult call to know when to move to mass production, and it should be noted the problems the USN post WWII attempts at austere escort ships (mobilisation prototypes in effect) had in being value for money in a peace time navy. The USN preferred any escorts be built to warship standards.
The RN approach was to built many escorts to merchant ship standards so the merchant shipyards could build them and then convert them post war into merchant ships. The bottom of the range escort was about as fast as a U-boat and slightly outgunning it. Escorts with better capabilities were built to warship standards. In this way the RN/RCN/RAN etc. moved from 32 escorts in September 1939 to 88 in December 1940 and 216 by December 1941 plus 10 ex Coast guard cutters. Despite the loss of some 15 escorts to the end of December 1941. Then add 19 Hunt class DEs in 1940 with 44 being in commission by the end of December 1941 (one was lost in 1941). Also there was the 50 ex US destroyers 3 of which had been lost in 1941. These gains were offset by the decline of the RN destroyer fleet (new and old) from 184 at the start of the war to 156 by the end of December 1941. A smaller second line escort could be built more quickly in more yards and not compete as much with the rest of the warship program, though of course it would then compete against the smaller ship programs like landing craft. Such a small ship would have poor AA (minimal fire control), poor anti surface abilities but excellent anti submarine capabilities. This would leave little margin for wartime or post war developments or other uses.
U-boat fleet size, from 57 in service in September 1939 to 73 in December 1940 to 236 in December 1941, including Arctic and Mediterranean forces, after 67 losses (counting U-31 as one not two losses)
The 1939/40 "USN view" was that the best second class escort was the current 1,620 (or 1,630) ton fleet destroyer, (Benson/Livermore classes) which was about to be replaced by the 2,100 ton Fletcher class as the fleet unit. This had the advantages of being in production, so no delay for final design, shipyards build repeat ships more quickly, it would be a very good AA ship with good anti surface ship capability and good anti submarine capability, since they were unlikely to be simply discarded post war (remembering the situation of WWI with all those 4 stackers) they would be better hulls for future developments. The down side was these ships would compete with the fleet destroyers and other heavy warships for steam turbines, gearing, fire control, weapons and slipways since they would need to be built in naval shipyards. They would also consume more manpower per unit. In the second half of 1940 the US ordered 204 destroyers, 119 Fletchers, 2 experimental types using Fletcher hulls and the rest the current 1,620 (or 1,630) ton standard destroyer. Some of these were actually laid down before the relevant fiscal year. These orders would double the existing USN destroyer fleet (new and old) and served to enable the upgrading of existing shipyards and even a new yard at Orange, Texas. No more destroyer orders were placed until after the US entered the war, a further 56 Fletcher class, then the Sumner and Gearing classes.
The old USN destroyers, there were 170 hulls of which 50 were handed over to the RN and 21 converted to fast transports and seaplane tender uses before December 1941, plus a further 25 converted to minelayers/sweepers. So in effect the nett result was a reduction from 170 escort hulls available to 74, unless the USN pressed the conversions into escort duties, 10 RN flower class corvettes were transferred to the USCG between March and May 1942.
In 1937 the USN started to revive interest in anti submarine ships, the resultant ship was the 173 foot PC, in effect a coastal escort to release the older destroyers for ocean escort work. In June 1939 the idea for an intermediate or second line type was proposed, somewhere between the sub chaser and the destroyer, "In many types of screening two boats, covering more ground than one, can be of more use than one, even if they are inferior in fighting strength". Admirals do not like to send "inferior" ships to sea. The General Board of the USN balked at the idea of a second rate ship and tried reworking the oldest "modern" destroyer, the Farragut class, but could not do it, so in January 1940 the persuasive argument was the current USN destroyers at 1,620 tons should be considered the standard for the mass produced "secondary" escort, especially with the appearance of the more expensive 2,000+ ton Fletcher class.
In mid 1940 Gibbs & Cox managed to convince FDR to order 4 of their lightweight destroyers, the USN quite sensibly managed to have the construction stopped, they lacked too many basic requirements. At the same time the USN created a 1,175 ton design to which the lightweight destroyer orders were amended to. There were at least 7 USN designed DE proposals between August 1940 and February 1941, the trend seems to be removing 5 inch guns and torpedo tubes to concentrate on anti submarine weaponry and to keep ship size and crew requirements down. The trouble with most of these designs was the USN was still trying for ships that required shipyards able to build destroyers. There was considerable to and fro over things like whether dual purpose guns were needed for AA fire (which triggered a bigger ship with the internal volume for the fire control systems and the ability to carry weights high in the ship), torpedo tubes to at least threaten enemy surface raiders and so on. The USN kept coming back to building more 1,620 ton destroyers as it's preferred option. On 16 May 1941 the Chief of Naval operations proposed the cancellation of orders for 50 escorts and this was done 3 days later. Two months later the secretary of the navy approved the Chief Of Naval Operations and USN General Board recommendation that no escorts be built given the delay in construction and the proposed and possible destroyer programs. Cost estimates also helped, since it was estimated a DE would cost $6.8 million versus $8.1 million for a 1620 ton Destroyer (in 1943 the figures for the DEs then being built were around $5.3 to $6.1 million versus $10.4 million for the 1,620 ton destroyer). This analysis failed to note that in wartime money becomes secondary to capacity.
Clarification came in the guise of a British order for 50 escort vessels, the BDEs, requested 23 June 1941, approved by FDR on 15 August 1941. By now the DE design had minimal overlap with the destroyers, the DEs could be built without steam turbines or 5 inch guns. Most DEs were built at yards that did not build destroyers. In the end the USN commissioned no DEs in 1942 and 234 in 1943. The first DEs were laid down in February 1942 and what is more interesting is their real competitor for resources was the LST, some 255 of which were laid down in 1942 versus 89 DE. Large numbers of LST orders were cancelled in 1942 to free resources for DE, then in 1943 large numbers of DE orders were cancelled to allow more LST. During the last half of 1940 the USN ordered as many destroyers as the US could build over the next two to three years. If it wanted more anti submarine capability in 1942 it needed to devise an escort that would not compete for resources very much with destroyers, the DE. The USN was largely opposed to such an idea. A key decision was not replacing the 50 old destroyers transferred in 1940 with an order for 50 ocean going escorts. Post war the DE were obsolete as soon as the faster submarines were placed in service though as it turned out the increase in submarine abilities meant post WWII the Fletcher class was the smallest hull suitable for anti submarine operations, the 1,620 ton destroyers were all quickly put out of service.
Limits on production are clear, you only have to look at the way DEs only started receiving 5 inch guns around when the Sumner and Gearing classes moved to the twin mounting, freeing the single mount production, the fact the average DE had 6 or 12,000 HP versus 50,000 in the 1620 ton destroyer and the mixture of propulsion units used, diesels (Evarts class, 97 units, Canon class, 72 units, Edsall class, 85 units), turbo electric (Buckley class 154 units, Rudderrow class 72 units) and turbines (Butler class geared turbines, 83 units). Simply not even the US could provide more geared turbines for a start (it took around a year to build the machine tools to cut the gears), plus the 5 inch guns and fire controls. The Rudderow and Butler classes had the 5 inch guns, these were in general the last DEs delivered, usually in 1944.
The Iowa class Illinois cancelled when 22% complete, the Kentucky when 73% complete, the Alaska class Hawaii cancelled when 84% complete. The last Iowa class to be completed was laid down on 25 January 1941, the two completed Alaska class on 17 December 1941 and 2 February 1942.
The Italians had around 120 submarines in June 1940, increasing their effectiveness and transferring most to the Atlantic would help the axis cause, as would using at least some of the 75 or so French submarines for training at least.
News of the type XXI prompted a wartime British program to streamline etc. some of their older submarines enabling more underwater speed to help work put tactics against faster submarines.